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There have been quite a few attempts made at derailing the thread - I'd suggest anyone who seeks to question the design or designer first read the articles and measurements on the O2. If you can't be bothered doing that, don't bother posting.
Looks like the revised board is complete. Pending measurements, and once the performance of the new layout is verified it'll be available to everyone - here's hoping vendors will jump on it quickly and provide some O2-for-dummies kits soon.
I will ask my friendly pcb company to quote for the board with the bom populated on Monday when most of them are back to work from their holidays, should be fun
And I wonder what parts (if any) will be on long lead times at the 1000+ qty , will find out soon I guess, this would probably suit most UK and European buyers, as the factory is based in England, so cheaper shipping/postage costs and no custom duties to worry about.
Bulk Mouser buys for the UK could be possible - with shipping at £12 several people lumping together their orders to get them over the £50 free-shipping mark would save a reasonable sum of money.
this is the DIY forum, not the SS. friendly banter and a sense of humor is the norm here.
And as you increase the gain, you are running more power through the pot. A worst case scenario of 2VRMS input at a gain of 7 through a 10k pot gives you 19.6mW. An Alps Blue rated at 50mW would be fine - but it is high for, say, a TKD 2CP which is rated at only 20mW.
I doubt it will be a significant problem, but certainly an odd decision to my mind.
Not so odd... the pot won't add Johnson noise pre-amplification (which would just be amplified as it is pulled through the VAS if the pot were on the I/P). Additionally, any noise created upstream of the pot has a chance of being attenuated by the pot; both cases reducing SNR going forward. It's how a lot of non-audio applications where high SNR is a requirement take on adjustable gain through an amp/pre-amp package.
did I read group buy for a kit with all the parts?
Not yet. The design isn't quite finalized yet.
There's no way you'd be able to get 14 Vrms into a volume pot with an amp running off of a +/-12 V supply. Maximum unclipped sine level at this point would be 7 Vrms and change, so we'd be closer to 5 mW. That's why the choice of two gain settings is handy, you can choose low gain such that there will be no clipping even with a hot sauce^Wsource (a few rare examples output up to 2.5 Vrms fullscale, and 2.2 Vrms is quite common among modern-day CD players), while high gain will accomodate weaker sources like portable players.
As mentioned, it does make sense to come reasonably close to volume pot power handling in worst-case conditions (let's say, up to 20% or so). SNR still is defined by the ratio of signal power and noise power. Noise power is fairly constant, if we assume it to be dominated by Johnson (thermal) noise (no clue what the excess noise properties of typical volume pots would be). Hence, you don't get around dumping a certain amount of power into the pot if maximum SNR is a priority. Choosing a 10k instead of a conventional 50k pot simply increases available signal power by a factor of 5, so worst-case SNR could be expected to be up to 7 dB better. (Doing the math with the following stage's voltage noise included, it would be more like 5 dB in practice.) And noisy this design definitely isn't, with an advantage over a single-stage architecture that grows with gain.
In order to avoid the first-stage clipping problem, one would have to employ a two-stage volume control, adding a second pot or stepped attenuator at the input.
Time to go kidnap a dynamic and fire up the beta22, I'll be back later tonight with some impressions.
Let's see if this thing is the giant killer it's made out to be.
Hopefully something more resolving than a HD600/650.
This is a real problem in this design, particularly on battery power. Dual 8.4V batteries are specified, let's say that batteries are charged and does not fall below the rated voltage, we have 16.8V total spread, minus the forward drop of the two 1N5818 schottky diodes (0.5V each), this reduces the available voltage down to 15.8V. And let's assume no further voltage drops in the power supply -- in fact there is some drop across the MOSFETs, but probably small enough to ignore for this discussion. The NJM2608 opamp is not rail-to-rail. It could only swing to about 2V above the negative rail and about 1V below the positive rail. The signal's negative peaks will therefore clip first and effectively we have a maximum output voltage swing of 11.8Vp-p.
The volume pot is not at the input to attenuate the input signal, so the input opamp "sees" the full output voltage from the source. Let's assume a standard Redbook audio CD player's output voltage at 0dBFS of 2Vrms, which is 5.7Vp-p. Even at the lowest gain setting of 2x, the opamp output will be swinging 11.3Vp-p which is right at the verge of clipping. If you switch the gain any higher, it will clip rather severely.
The designer states on the schematic "Input 4V RMS max". If that was true, we'd need to be able to swing 22.6Vp-p at the output of the opamp for a gain of 2x (and more for higher gains). Neither a battery-powered nor a wall-powered version of this amp could do that without clipping.
This is irrespective of the headphone sensitivity or volume pot position. While similar concepts have been used in commercial gear, the input stage in them would be powered by much higher supply voltages in order to avoid clipping, but that is clearly not an option here due to the batteries.